Polysiloxane-containing thermosetting solution coating compositions are known in which the polysiloxane component is condensed with a reactive resin to provide a combination which retains its reactivity and which is combined with a curing agent, such as a melamine-formaldehyde condensate, so that the coating can be applied and thermally cured upon a metal substrate. In these known compositions, an excessive proportion of polysiloxane is needed to achieve the desired weather resistance.
It is also known to use a polysiloxane which is not reactive so that it stratifies and concentrates at the surface of the coating during the cure. However, these polysiloxanes volatilize and burn during the baking operation, forming silica particles which deposit in the oven. This is undesirable and wasteful of the expensive polysiloxane component. Also, inadequate compatibility in solution causes these systems to have poor package stability
In this invention, we minimize the problems encountered by the prior art by having the polysiloxane component in a nonvolatile combination which still permits controlled incompatibility to develop during cure to concentrate the polysiloxane at the surface and thereby improve the weather resistance which is obtainable, using a given proportion of polysiloxane.